Add the suite of Mock API serialization tests to the C# SDK.

[armstnp]

There were five serialization tests that failed in various ways, and are thus currently configured to be ignored:

BulkFailure
The failure for this one is pretty mysterious, as the JSON deserializer seems to expect more content, but it's unclear as to which content it expects and isn't seeing.

Predecessor
This has two points of failure: AddRows should support query parameters (include specifically, here), but does not; and the generic ObjectValue type is not deserialized from the response, currently. This test will need to be unignored and its assertions uncommented to be considered fully active once fixed.

Sheet Settings
The Schedule object's 'Non-working days' field expects day-level granularity dates ('2018-10-10', e.g.), but the SDK always serializes it to full date-times.

Update Request
The Schedule object's lastSentAt and nextSendAt fields ought to be optional, but instead are mandatory and sent with every request, instead of only being received by a Schedule response.

User
The profileImage field expects a response ID named imageId, not id, and so deserialization fails.

[armstnp]

Added tests for 'Change Agent' and 'Assume User' headers.